EP4332272A1

EP4332272A1 - Solution

Info

Publication number: EP4332272A1
Application number: EP22386059.4A
Authority: EP
Inventors: Ioannis Kontonis
Original assignee: Individual
Current assignee: Individual
Priority date: 2022-08-31
Filing date: 2022-08-31
Publication date: 2024-03-06
Also published as: WO2024047136A1

Abstract

The present invention relates to a continuous process for preparing a stable hypochlorous acid solution by electrolysis and an apparatus for carrying out same.

Description

Field

The present disclosure relates to an ultrapure and stable hypochlorous acid (HOCI) solution, a process and apparatus for its production and various applications for the use of the solution.

Background

Hypochlorous acid and its production have been known for decades. It is widely recognised in the art as an effective antimicrobial agent with activity against bacteria, viruses, fungi and spores. It is particularly favoured for use in medical, veterinary, agricultural and industrial uses. There are, however, numerous problems with the processes for preparing hypochlorous acid known in the art.
HOCI is a weak acid that exists when chlorine is dissolved in water. In aqueous solution, HOCI partially dissociates resulting in the formation of the anion hypochlorite OCI- as shown below:

Cl ₂ + H ₂ O ↔ HOCl + H ⁺ + Cl^-

HOCl ↔ H ⁺ + OCl
When acids are added to aqueous salts of hypochlorous acid, such as sodium hypochlorite, the top equilibrium is therefore driven to the left and chlorine gas is formed. Conversely, the formation of hypochlorite bleaches is facilitated by dissolving chlorine gas into basic water solutions, such as sodium hydroxide. Until recently, sodium hypochlorite was therefore the principal form in which hypochlorous acid was available. When sodium hypochlorite reacts with water, hypochlorous acid is formed:

NaOCl _(aq) + H₂O_(l) → HOCl _(g) + NaOH _(aq)
As hypochlorous acid forms as a gas in the reaction, it quickly dissipates. It also reverts to sodium hypochlorite when it reacts with the alkaline solution. Consequently, the hypochlorous acid prepared by this method is highly unstable.
Another known method is to prepare HOCI solutions in situ using electrolysis of a solution of brine (NaCI). This requires on-site apparatus to prepare the solution, with the resulting high cost associated with both the preparation of the HOCI solution and processing of the inactive waste solution with a short half-life. The electrolysis is carried out in commercially available equipment, typically with an electrolysis double chamber - one chamber containing the anode and another containing the cathode - and a permeable membrane. Electrolysis of brine is a slow process which is open to variation, and the resulting HOCI solution is not overly stable. The hypochlorous acid produced is inevitably highly acidic, pH 2.0-3.0, and this results in the presence of chlorine gas which is not desirable for many reasons.
Conventional electrolysis equipment is also problematic because of the permeable membranes contained therein. These membranes are often called "ion membranes" or "separating membranes" and are expensive and easy to break during the electrolysis process, hence affecting the process efficiency. Whilst membrane-free electrolysis devices are known, they are not typically associated with the production of an ultrapure and stable hypochlorous acid solution as in the present disclosure.
Other known methods for preparing hypochlorous acid include that described in GB 2488838 A . Here a calcium hypochlorite is dissolved in aqueous solution where the calcium hypochlorite has a high chlorine to chloride ratio. This ratio means that an alkaline solution of calcium and hypochlorite ions is formed. The calcium is removed, e.g. by filtration of a calcium hydroxide precipitate, and the pH is adjusted using phosphoric acid so that calcium phosphate is formed and subsequently removed by filtration. The product is described as "a near pure solution of hypochlorous acid" but the pH is still adjusted using further phosphoric acid or sodium hydroxide. This will contaminate the end-solution making it unsuitable for therapeutic use. Contaminants are known to cause side-effects like eye or skin irritation, sensitivity, allergic reactions and lung irritation.
Another preparation method for hypochlorous acid is described in Wang et al.., J. Burns Wounds, 2007, 6, 65-79. Hypochlorous acid is prepared in brine solution by acidifying reagent-grade NaOCI to the pH range of 3.5 to 4.0 with dilute HCI. The resulting solution contains bleach and hypochlorous acid salts as contaminants. A buffer is also required to maintain the pH.
WO 2010/148004 describes a process in which chlorine gas is added to a buffer solution containing buffering agent and water. Also disclosed is an electrolysis process involving the use of a commercial apparatus as outlined above. Specifically, the apparatus has an anode chamber, a cathode chamber and a salt solution chamber located between the anode and cathode chambers. The solutions have the disadvantage of comprising bleach molecules and needing to be buffered for stability, potentially resulting in further contamination. The apparatus has a low service life and the production cost of the product is high.
There are obvious consequences of adding other components to a hypochlorous acid solution. One is an unavoidable and undesirable increase in dissolved solids content, others include safety considerations for the added components. The latter is particularly key when the hypochlorous acid solution is intended for use in a medical or veterinary setting. The solution is not "pure" hypochlorous acid, but a mixture and often an equilibrium between hypochlorous acid and additive. It would be desirable to have an "ultrapure" and stable hypochlorous acid solution. This is not currently provided by the prior art. It would also be desirable to provide an electrolysis apparatus that has improved efficiency, lower running costs, and a longer service life. This is similarly not provided by commercially available, known apparatus.
The above objects are satisfied by the present disclosure. In particular, the process of the present disclosure prepares a hypochlorous acid solution which allows hypochlorous acid to be manufactured in an ultrapure form in which it is highly stable in an aqueous solution. The terms "ultrapure" and "stable" are defined herein. The process generates defined and specific concentrations of the ultrapure, stable hypochlorous acid solution at a defined range of pH values and thereby is an entirely controllable process. Notably the solutions do not require the addition of buffer or any other component to achieve stability and are not reliant on bleach for their production.

Summary

In one aspect, there is provided a continuous process for preparing a stable hypochlorous acid solution by electrolysis. The process comprises: providing a hydrochloric acid feed at a defined flow rate, providing a water feed at a defined pressure, adding the hydrochloric acid feed to the water feed to form a reactant solution feed and providing the reactant solution feed to a chamber of an electrolysis cell, the chamber comprising an anode and a cathode wherein at least one is platinum-coated, and applying a voltage to the electrolysis cell to generate a defined current.
The process further comprises measuring the pH of the reactant solution before the reactant solution feed is provided to the single chamber electrolysis cell, and measuring the pH of the hypochlorous acid solution as it exits the single chamber electrolysis cell.
The flow rate of the hydrochloric acid feed is about 2 ml/minute to about 6 ml/minute. The pressure of the water feed is about 7 psi to less than about 15 psi. The current of the cell is about 5A to about 20A. The electrical current, the hydrochloric acid feed flow rate, the water feed pressure and the reactant solution feed flow rate are monitored in real-time and controlled so that the pH of the reactant solution is up to about 3.5 and the pH of the hypochlorous acid solution is from about 4.0 to about 6.0. Preferably the electrical current, the hydrochloric acid feed flow rate, the water feed pressure and the reactant solution feed flow rate are controlled so that the pH of the hypochlorous acid solution is at a constant value from about 4.0 to about 6.0. The term "constant" meaning ±0.5 pH, preferably ±0.2 pH.
In another aspect, there is provided a hypochlorous acid solution obtained by the process defined herein.
In another aspect, there is provided an apparatus for preparing a stable hypochlorous acid solution. The apparatus may be used to implement the process defined herein. The apparatus comprises a single chamber electrolysis cell containing an anode and a cathode, at least one of the anode and the cathode being platinum-coated. The apparatus further comprises a water conduit and a hydrochloric acid conduit configured to provide a reactant solution feed to the single chamber electrolysis cell. The hydrochloric acid conduit is connected to the water conduit via a first non-return valve, and the reactant solution feed is provided by a conduit connected to the single chamber electrolysis cell via a second non-return valve. The apparatus further comprises a conduit for hypochlorous acid solution, a pH meter connected to the reactant solution conduit, a pH meter connected to the hypochlorous acid solution conduit and a controller.
The controller is electrically connected to the electrolysis cell, the water conduit, the hydrochloric acid conduit, the reactant solution conduit, and each of the pH meters. The controller includes a means to monitor, in real-time, the electrical current of the electrolysis cell, the hydrochloric acid flow rate, the water pressure, the reactant solution flow rate, the reactant solution pH and the hypochlorous solution pH, and a means to control one or more of the electrical current, the hydrochloric acid flow rate, the water pressure and the reactant solution flow rate so that the pH of the reactant solution is up to about 3.5 and the pH of the hypochlorous acid solution is from about 4.0 to about 6.0. Preferably the electrical current, the hydrochloric acid feed flow rate, the water feed pressure and the reactant solution feed flow rate are controlled so that the pH of the hypochlorous acid solution is at a constant value from about 4.0 to about 6.0. The term "constant" meaning ±0.5 pH, preferably ±0.2 pH.
These aspects and embodiments thereof are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and with features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the present disclosure is not restricted to the specific embodiments set out below, but includes and contemplates any combinations of features presented herein.
The foregoing and other aspects, embodiments, features and advantages of the present disclosure will be apparent from the following detailed description along with the accompanying drawings. In this regard, particular sections of the description are not to be read in isolation from other sections. It is also expressly noted that the drawings are for illustrative purposes only, they are not to be construed as defining the limits of the present disclosure.

Brief Description of the Drawings

Figure 1 is a distribution curve of chlorine species against pH.
Figure 2 is a schematic representation of an exemplary apparatus of the present disclosure.
Figure 3 is a schematic representation of the front panel of an exemplary controller according to the present disclosure.
Figure 4 is a schematic representation of an exemplary single cell electrolysis chamber and an exemplary cooling system.
Figure 5 is an exemplary arrangement of the hydrochloric acid feed being added into the water feed to provide the reactant solution in the process of the present disclosure. Figure 5 also demonstrates an exemplary arrangement of the first and second non-return valves in the apparatus of the present disclosure.

Detailed Description

While various exemplary embodiments are described or suggested herein, other exemplary embodiments utilizing a variety of methods and materials similar or equivalent to those described or suggested herein are encompassed by the general inventive concepts. Those features or embodiments which are implemented conventionally may not be discussed or described in detail in the interests of brevity. It will thus be appreciated that features of apparatus, products or processes described herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such features in the respective context.
The general inventive concept is centred on using hydrochloric acid and water in a continuous electrolysis process to generate hypochlorous acid in an ultrapure and stable form. The chemical reaction that occurs is shown below:

2HCl + H ₂ O → HCl + HOCl + H ₂(gas)
Both water and hydrochloric acid as raw materials are widely available and low cost. In addition, the use of water and hydrochloric acid is beneficial because there is no toxic waste stream. The inventive concept is further centred on an apparatus for the continuous electrolysis process. The apparatus has no permeable or separating membrane or diaphragm (unlike the commercially available electrolysis machines), a platinum-coated anode and/or cathode and a single electrolysis chamber meaning that it is simple to operate, highly efficient and has a long service time. In addition, the preparation of the hypochlorous acid solution by a single electrolysis chamber means that there are no complicated or complex downstream steps following electrolysis. The ultrapure and stable hypochlorous acid solution can be used directly from the process of the present disclosure. The process of the present disclosure is particularly unique in that the electrical current, the hydrochloric acid feed flow rate, the water feed pressure and the reactant solution feed flow rate are monitored in real-time and controlled so that the pH of the reactant solution before electrolysis and the pH of the resulting hypochlorous acid solution are within a defined range.
The hypochlorous acid solution obtained by the process of the present disclosure is ultrapure and stable. By the term "ultrapure" is meant free from both chlorine gas and hypochlorite ions. In various embodiments, the solution comprises less than about 2%, preferably less than about 1%, more preferably less than about 0.5%, most preferably less than about 0.01% chlorine gas. The solution similarly comprises less than about 2%, preferably less than about 1%, more preferably less than about 0.5%, most preferably less than about 0.01% hypochlorite ions.
The chlorine gas and hypochlorite ions are measured as a percentage of the total free available chlorine, i.e. FAC.
In various embodiments, the solution comprises less than about 2% chlorine gas and less than about 2% hypochlorite ions. In various embodiments, the solution comprises less than about 1% chlorine gas and less than about 1% hypochlorite ions. In various embodiments, the solution comprises less than about 0.5% chlorine gas and less than about 0.5% hypochlorite ions. In various embodiments, the solution comprises less than about 0.01% chlorine gas and less than about 0.01% hypochlorite ions.

It is known in the art that FAC is a combined form of hypochlorous acid, hypochlorite anion and dissolved chlorine gas in aqueous solution. The chlorine specification profile is shown in Figure 1 and the FAC% is shown in Table 1 below. It can be seen how lower pH values favour the presence of chlorine gas and higher pH values favour the presence of hypochlorite ion.

pH of electrolysed solution	FAC % composition
pH of electrolysed solution	HOCl	Cl₂	OCl^-
2.7	70	30	0
3.0	80	20	0
3.5	90	10	0
4.0	95	5	0
4.5	100	0	Trace
5.0	100	0	Trace
5.5	100	0	Trace
6.0	98	0	2
6.5	95	0	5
7.0	7.8	0	22
7.5	50	0	50

The total chlorine and %FAC is measurable with a chlorinometer. For example, the HI 96771 Hanna Instruments UHR photo chlorinometer.
By the expression "stable hypochlorous acid solution" is meant the concentration of hypochlorous acid in the solution does not change by more than ±10 ppm and the pH does not change by more than ±0.2 pH when the solution is stored in a dark glass container at 30°C±2 °C and 65%±5% RH. The hypochlorous acid solution of the present disclosure is generally stable for at least 3 months, preferably at least 6 months, more preferably at least one year, most preferably at least 15 months. The material of the container is not critical and other dark containers known in the art may be used. For example, a dark polyethylene terephthalate (PET) or other moisture resistant plastic container may be used.
In various embodiments of the present disclosure, the concentrations of free available chlorine species are maintained at about ±8% from their initial concentration for at least 12 months after preparation of the hypochlorous acid solution. Preferably, these concentrations are maintained at about ±5% from their initial concentration for at least 12 months after preparation of the hypochlorous acid solution.
The stable nature of the HOCI solutions enable the solutions to be provided in a variety of different formats for their appropriate delivery to a site for use. A wide range of applications is also possible. For example, these may include, but are not limited to cosmetic preparations (e.g. gels, creams mouthwash solutions, wipes, after-sun products), medical device preparations (e.g. wound care gels and lotions, impregnated gauzes, throat gargle solutions, eye relief solutions), detergent preparations (e.g. disinfecting cleaning products, washing liquids, laundry liquids, fabric disinfection products), potable water biocides, veterinary use biocides and industrial use biocides. Methods of delivery of the solution may also vary and include pouring, injection, pumping, immersing, coating, spraying, misting and fogging. sanitizing or sterilising hard surfaces, living organisms and environments. In particular, the solution may be used as an antimicrobial agent. If a specific concentration is desired, it is possible to dilute the ultrapure hypochlorous acid solution prepared by the process of the present invention with an appropriate amount of sterile water.
The pH of the hypochlorous acid solution of the present disclosure is from about 4.0 to about 6.0, preferably from about 4.5 to about 6.0, more preferably from about 4.5 to about 5.7. The solution does not include a buffering agent to alter the pH. The term "buffering agent" is well-understood in the art and may be used interchangeably with "buffer" or "pH buffer". Examples of buffering agents include citrate, acetate, adipate, succinate, borate, formate, benzoate, carbonate, propionate, phosphate, salts thereof and combinations thereof.
The concentration of hypochlorous acid in the solution is at least about 80 milligrams per litre at a pH from about 4.0 to about 6.0, preferably from about 4.5 to about 6.0, more preferably from about 4.5 to about 5.7. In the present disclosure, the units "milligrams per litre" are used interchangeably with "parts per million" or "ppm". The concentration of hypochlorous acid in the solution is preferably at least about 80 ppm to about 140 ppm at a pH of about 4.0 to about 6.0, preferably from about 4.5 to about 6.0, more preferably from about 4.5 to about 5.7. More preferably the concentration of hypochlorous acid in the solution is from about 90 ppm to about 140 ppm, e.g. from about 100 ppm to about 140 ppm or from about 120 ppm to about 140 ppm, at a pH of about 4.0 to about 6.0, preferably from about 4.5 to about 6.0, more preferably from about 4.5 to about 5.7. In each of these embodiments, the solution does not include buffering agent other than the hypochlorous acid.
In one embodiment the concentration of hypochlorous acid in the solution is from about 100 ppm to about 140 ppm at a pH of about 4.0 to about 6.0, preferably from about 4.5 to about 6.0, more preferably from about 4.5 to about 5.7. In each of these embodiments, the solution does not include buffering agent other than the hypochlorous acid.
In one embodiment the concentration of hypochlorous acid in the solution is from about 120 ppm to about 140 ppm at a pH of about 4.0 to about 6.0, preferably from about 4.5 to about 6.0, more preferably from about 4.5 to about 5.7. In each of these embodiments, the solution does not include buffering agent other than the hypochlorous acid.
In one embodiment, the stable hypochlorous acid solution has a concentration of hypochlorous acid of 100 milligrams per liter and a pH of about 5.1. The solution is free of buffering agent. In one embodiment, the stable hypochlorous acid solution has a concentration of hypochlorous acid of 140 milligrams per liter and a pH of about 4.8. The solution is free of buffering agent.
The solutions of the present disclosure are also free of stabiliser, namely any compound or component added for the purpose of stabilising the hypochlorous acid. Examples are known in the art and include e.g. sodium chlorate and sodium hypochlorite. The lack of additional stabilisers is desirable for handling and downstream application and use of the hypochlorous acid. The process of the present disclosure is free from sodium hypochlorite and free from sodium chlorate.
As noted above, the process of the present disclosure is unique in that the electrical current, the hydrochloric acid feed flow rate, the water pressure and the reactant solution feed flow rate are monitored in real-time and controlled so that the pH of the reactant solution prior to electrolysis and the pH of the resulting hypochlorous acid solution are within respectively defined ranges. In particular, the pH of the reactant solution is up to 3.5 and the pH of the hypochlorous acid solution is from about 4.0 to about 6.0. The term "real-time" would be readily understood by the person skilled in the art, it means as the continuous process is being carried out. The process is fully controllable and, unlike the processes of the prior art, prepares a stable, ultrapure hypochlorous acid solution in a continuous manner.
The pH of the reactant solution and the pH of the hypochlorous acid solution are measured by using pH meters as schematically shown in Figure 2. Such pH meters are known in the art. One such meter is, for example, available from Mettler Toledo. The pH meter is preferably in-line as exemplified by Figure 2. The measured pH is then monitored using a controller as schematically shown in Figure 3.
An in-line pH meter is preferred because it provides an instant and continuous pH measurement as the process is carried out.
The hydrochloric acid feed flow rate is monitored and controlled in real-time. The hydrochloric acid feed flow rate is, in particular, about 2 ml/minute to about 6 ml/minute. Preferably about 2 ml/minute to about 4 ml/minute. More preferably about 3 ml/minute to about 4 ml/minute. The hydrochloric acid flow rate may be monitored and controlled with an electronically controlled dosing pump and the controller described herein.
The hydrochloric acid is added to the water feed to form a reactant solution. As the process is continuous and not a batch process, the hydrochloric acid is continuously added into the water feed. Preferably the hydrochloric acid is injected dropwise to the water feed. This may be achieved by using the dosing pump with a non-return valve. Such an arrangement is demonstrated by Figure 5. More preferably the hydrochloric acid is injected dropwise at a set pressure and flow rate into the water feed. The hydrochloric acid feed flow rate is defined above. This pressure may be controlled via the use of multiple non-return valves in combination with the dosing pump as demonstrated by Figure 5. In particular, a non-return valve is adjacent to each of the inlet and outlet of the dosing pump. In this manner, the pressure of the pumped hydrochloric acid is maintained at a set level and thereby achieves high accuracy injection into the water feed. The pressure of the hydrochloric acid maintained by the non-return valves and dosing pump may be between 2 and 20 psi. Preferably the pressure of the hydrochloric acid injection may be between 2 and 15 psi. More preferably the pressure of the hydrochloric acid injection may be between 2 and 10 psi.
The hydrochloric acid feed pressure will be lower than the water feed pressure.
The water pressure is monitored and controlled in real-time in the process of the present disclosure. The water pressure is about 7 psi to less than about 15 psi. Preferably about 7 psi to about 12 psi. More preferably about 10 psi to about 12 psi. The water pressure may be monitored and controlled by a pressure regulator as shown in Figure 2 and the controller defined herein.
The reactant solution feed flow rate is monitored and controlled in real-time in the process of the present disclosure. The reactant solution feed flow rate is, in various embodiments of the present disclosure, about 0.4 to about 2.5 l/min. Preferably about 0.4 to about 1.5 l/min, more preferably about 1 to 1.5 l/min. The reactant solution flow rate may be monitored and controlled by a flow meter and the controller defined herein.
The water feed flow rate is, in various embodiments, monitored and controlled. It is typically found to be similar if not identical to the reactant solution flow rate. This is because the hydrochloric acid is provided at a relatively low flow rate - about 2 ml/minute to about 6 ml/minute - meaning that the reactant solution flow rate is not believed to be affected in terms of liter/minute by the addition of hydrochloric acid. In preferred embodiments the water flow rate is about 0.4 to about 2.5 l/min. Preferably about 0.4 to about 1.5 l/min, more preferably about 1 to 1.5 l/min. The water feed flow rate may be monitored and controlled using a flow meter and the controller in combination with the pressure regulator noted above.
The monitoring of water feed pressure, electrical current, hydrochloric acid feed flow rate, reactant solution feed flow rate, pH of reactant solution and pH of hypochlorous acid solution, and control of water feed pressure, electrical current, hydrochloric acid feed flow rate, and/or reactant solution feed flow rate allows full control of the process and the preparation of ultrapure and stable hypochlorous acid solutions as defined herein. This is demonstrated by the Examples herein. The controller is described below for the apparatus of the present disclosure.
The quantity of hypochlorous acid produced is advantageous. In various embodiments the quantity of hypochlorous acid produced is between about 20 and about 500 l/h. In various embodiments the quantity of hypochlorous acid produced is between about 20 and about 250 l/h. In various embodiments the quantity of hypochlorous acid produced is between about 20 and about 120 l/h. Preferably the quantity of hypochlorous acid produced by the process of the present disclosure is between about 24 and about 90 l/h.
The hydrochloric acid, as reactant, is a liquid and may be in aqueous solution. Typically the hydrochloric acid concentration in such a case may be between about 3.5% to about 10%, preferably about 3.5% to about 9%, more preferably about 3.5% to about 6%, for example about 4%. These concentrations may be measured by a hydrometer measuring the specific gravity of the solution at 20°C. As would be known to a person skilled in the art, there is a standard chart of the specific gravity of different concentrations of hydrochloric acid at different temperatures.
The hydrochloric acid is, preferably, chemically pure, food grade hydrochloric acid. In this form, the hydrochloric acid may be substantially free of contaminants such as metals or metal-based substances, and toxic substances like arsenic. Chemically pure, food grade hydrochloric acid is commercially available from sources known to the person skilled in the art. For example, Sigma Aldrich.
The water used as the reactant may be from any suitable water supply. The water may be tap water. Alternatively, the water may be pre-treated water including purified water, distilled water and deionized water. Preferably the pre-treated water source is ultrapure water obtained using reverse osmosis purification equipment. The water preferably comprises less than 2.5 mg/l calcium and/or less than 0.1 mg/l phosphate.
Following its preparation, the hypochlorous acid solution prepared by the process of the present disclosure may be transferred to a sealed container. The sealed container is dark but may be any suitable container to maintain the sterility and stability of the solution. Preferably the container is constructed of plastic or glass. The plastic may be rigid so that the container is capable of being stored on a shelf. Suitable plastics include polypropylene, polyethylene terephthalate (PET), polyolefin, cycloolefin, polyethylene, polyvinyl chloride, and mixtures thereof. Preferably the container comprises polyethylene selected from the group consisting of high-density polyethylene, low-density polyethylene, and linear low-density polyethylene. Most preferably, the container is glass, high density polyethylene or polyethylene terephthalate.
The apparatus of the present disclosure includes a single chamber electrolysis chamber with an anode and a cathode, at least one of the cathode and anode being platinum-coated. Preferably both the anode and cathode are platinum-coated. More preferably the anode and cathode are platinum-coated titanium alloy electrodes. As already noted herein, the inclusion of a single chamber electrolysis cell differs from many electrolysis devices currently used for hypochlorous acid production because such devices are known to have an anode chamber and a cathode chamber with a mixing tank either therebetween or downstream thereof. This results in a complex and inefficient device with a poor service life.
In particular, the apparatus includes a housing for the electrolysis cell. The electrolysis cell is then a sealed unit where the anode and cathode are plates made of a suitable material with at least one being platinum-coated. Preferably both the anode and cathode are platinum-coated. More preferably the anode and the cathode are plates made of titanium alloy and coated with platinum. In one example, the anode and/or cathode are copper-titanium-platinum electrodes.
The inclusion of an anode and/or cathode which is platinum-coated is a further benefit of the present disclosure. The use of platinum, and particularly titanium and platinum electrodes, are particularly advantageous over electrodes previously used in the art. In particular when compared to iridium or ruthenium-based electrodes, the advantages include: increased resistance to the acid solution, i.e. longer service life, an improved control of the direct current that passes through (due to the lower value of the dielectric constant (or relative permeability of Pt, compared with Ir or Ru), increased capacity in terms of the current that can pass through, excellent cost/efficiency ratio, and improved product quality.
The anode and cathode are at least partially and preferably fully immersed in the reactant solution fed into the chamber. Voltage is applied to generate an electrical current between the anode and cathode and drive the electrolysis reaction. The reactant solution in the chamber of the electrolysis cell preferably consists of, as reactants, the hydrochloric acid and water.
The electrical current driving the electrolysis reaction is, as noted above, monitored and controlled in real-time. The current is preferably monitored by an ampere meter and varied with a potentiometer. The electrical current is about 5 A to about 20 A, preferably about 5 A to about 15 A, more preferably about 10 A to 10 A. A current of such magnitude is passed, in use, between the electrodes at least when they are partly immersed and preferably fully immersed in the reactant solution of hydrochloric acid and water. The current is preferably direct. The potential difference, i.e. voltage, between the electrodes is typically up to about 24 volts. Figure 2 includes an example figure of 18 volts.
The electrolysis chamber has an input for the reactant solution feed and an output for the prepared HOCl. These are illustrated by Figures 2 and 4. Both the input and output are connected to the respective conduit and separated from one another. The input is provided at one end of the electrolysis cell and the output is provided at the other end of the electrolysis cell. In a preferred embodiment, the input for the reactant solution feed and the output for the HOCI solution are the only input and output of the single chamber electrolysis cell. This is the arrangement shown in each of Figures 2 and 4. In a more preferred embodiment, the input for the reactant solution feed is adjacent to the cathode of the electrolysis cell and the output for the HOCI solution is adjacent to the anode of the electrolysis cell. The single chamber electrolysis cell is preferably horizontal when in use. This is shown in Figure 4.
The hydrochloric acid and water are fed continuously into the electrolysis chamber to provide, at steady state, a steady volume of reactant solution in contact with the anode and cathode whilst the electrical current is continuously passed therebetween. This means that hypochlorous acid is prepared continuously by the process of the present disclosure. Preferably the addition of the hydrochloric acid to the water is electronically controlled, namely by the dosing pump described herein.
An electrical current of predetermined magnitude is passed between the two electrodes being immersed in the reaction mixture to drive the electrolysis. The current is preferably direct. Passing the electrical current between the electrodes is also, preferably computer controlled. The conductivity of the bulk reactant mixture should not vary due to the accurate control provided by the apparatus.
The apparatus includes a water conduit and a hydrochloric acid conduit configured to provide a reactant solution feed to the single chamber electrolysis cell. The hydrochloric acid conduit is connected to the water conduit via a first non-return valve, and a second non-return valve is used to connect the reactant solution conduit to the electrolysis cell chamber. This arrangement is shown in Figure 2. The use of multiple non-return valves is beneficial because they ensure that the pressure and therefore also the flow is maintained at a set level between them. This is advantageous because it provides a smooth and dropwise injection of hydrochloric acid to water and subsequent flow of reactant solution to the electrolysis chamber. As noted above, accurate control is thus provided by the apparatus of the present disclosure.
The apparatus of the present disclosure further comprises a controller which is electrically connected to the electrolysis cell, the water conduit, the hydrochloric acid conduit and the reactant solution conduit. The controller includes a means to monitor in real-time, the electrical current of the electrolysis cell, the hydrochloric acid flow rate, the water pressure, the reactant solution flow rate, and the pH of the reactant solution and HOCI solution. Also included is a means to alter one or more of the electrical current, the hydrochloric acid flow rate, the water pressure and the reactant solution flow rate in order to control the pH values within the defined ranges.
Figure 3 is a schematic diagram of the controller. The real-time monitoring and control facilitated by the controller is advantageous because it enables real-time and therefore accurate control of the pH of the reactant solution and HOCI and thus the concentration of HOCI in the end product. The controller includes circuitry and electrical components for the monitoring of the various parameters discussed herein, namely the electrical current, the hydrochloric acid feed flow rate, the water pressure, the reactant solution feed flow rate, the pH of the reactant solution and the pH of the HOCI solution. The controller also includes electrical components for the control of the electrical current, the hydrochloric acid feed flow rate, the water pressure and the reactant solution feed flow rate. The controller includes computer-implemented firmware developed by the inventor that can monitor and control these parameters and in various embodiments, a touch screen through which the user can control the appropriate parameters. This is labelled as "Programmable Logic Controller" in Figure 3.
The controller may further monitor the levels of reactants and product in and out of the single chamber electrolysis cell. Figure 3, for example, shows three "level indicators" on the controller. These level indicators in this embodiment refer - left-to-right- to HCI, water, and HOCI. In one embodiment the controller may include means to indicate to a user, e.g. a warning sign such as a flashing light or audible signal or a combination of both, that one or more reactant levels is below a predetermined level. In one embodiment the controller may include means to indicate to a user, e.g. a warning sign such as a flashing light or audible signal or a combination of both, that the HOCI solution is above a predetermined level in a vessel attached to the conduit of the apparatus. The controller may include both means of user indication. The controller may further comprise an automatic shutdown means should the warning sign not be responded to by the user after a predetermined time period.
In various embodiments, the electrolysis cell may have a cooling system. The cooling system and its location in the apparatus is not particularly limited and may include any suitable means to cool the cathode and anode in the electrolysis cell thereby reducing their temperature to avoid affecting the HOCI output. In one embodiment, the cooling system is directly associated with and external to the electrolysis cell as shown in Figure 4. Specifically the embodiment of Figure 4 has a "cold air input" which may be a tube or conduit supplying the single chamber electrolysis cell with cold air. The term "cold" is used to refer to a temperature which is below ambient, i.e. below 20°C. The person skilled in the art will be aware of other suitable cooling systems.
Having generally described the various aspects and embodiments thereof of the present disclosure, a further understanding can be obtained by reference to certain specific examples set out below which are provided for illustration only. These examples are not intended to be exhaustive or limiting.

Examples

Example 1: Preparation of ultrapure and stable hypochlorous acid solution

As already noted herein, it is important that the hypochlorous acid solution is generated at the correct pH. This is possible using an apparatus as shown in Figures 2, 3 and 4. Of note is that this apparatus does not include a "standard electrolysis chamber". It does not have a separating/permeable/ion membrane/diaphragm and includes the anode and cathode in a single electrolysis chamber for at least partial immersion in the reactant solution. The anode and cathode in this Example were platinum-coated titanium dioxide electrodes.
Using this apparatus, the inventors have found that it is possible to manufacture up to 140 mg/l hypochlorous acid at a pH in the range of 4.5 to 5.7.

In an exemplary process with the apparatus shown in Figure 2, 2 ml of hydrochloric acid having a concentration of 4% w/w was added dropwise to a water feed to obtain a reactant solution. This solution was added to the single electrolysis chamber and the electrolysis was carried out with monitoring and control of the parameters as shown in the table below, to obtain the recited hypochlorous acid solution. The reactant solution pH was between 2.5 and 3.5. The pH of the HOCI solution was between 4.5 and 5.7.

Reaction Conditions
Current (A)	Water Feed Pressure (psi)	HCl feed flow (ml/min)	Reactant Solution flow (l/min)	HOCI produced (ppm)	HOCI quantity produced (l/h)
5	7	2	0.4	80	24
10	10	3	1	100	60
15	12	4	1.5	140	90
20	15	6	2.5	90	120

It can be seen how controlling the electrical current, water pressure, hydrochloric acid flow rate and reactant solution flow rate resulted in an ultrapure hypochlorous acid solution being prepared at a satisfactory quantity. Notably it was found that the defined reactant solution flow rate coupled with the defined water pressure, the defined HCI flow rate and electrical current enabled the production of 140 ppm HOCI solution at 90 l/h. A higher water pressure, e.g. of 30 psi, has been recommended in the art to allow the user to manipulate the water supply pressure and subsequent flow rate. The modification of electrical current was also surprising since lower levels of current, e.g. between 1A and 2A, have been typically used for electrolysis reactions in the prior art.

Example 2: Stability testing

The stability of hypochlorous acid solutions prepared by the process of Example 1 was tested under ICH Conditions of 30°C±2 °C and 65%±5% RH. The tested solutions were as follows:

i) 80 mg/l HOCI, pH 5.4 stored in unopened 10 ml dark glass bottles with glass dropper dispenser at 30°C±2 °C and 65%±5% RH.
ii) 100 mg/l HOCI, pH 5.1 stored in unopened 10 ml dark glass bottles with glass dropper dispenser at 30°C±2 °C and 65%±5% RH.
iii) 140 mg/l HOCI, pH 4.8 stored in unopened 10 ml dark glass bottles with glass dropper dispenser at 30°C±2 °C and 65%±5% RH.

The stability results are shown in the Table below.

	Concentration ppm (as FAC)
	80		100		140
Month	ppm	pH	ppm	pH	ppm	pH
0	80	5.4	100	5.1	140	4.8
1	80	5.4	100	5.1	140	4.8
2	78	5.5	100	5.1	140	4.8
3	79	5.5	99	5.0	140	4.8
4	78	5.6	98	5.0	135	4.7
5	79	5.4	97	5.0	135	4.7
6	79	5.5	98	5.0	134	4.7
7	79	5.5	97	5.0	133	4.7
8	77	5.5	97	5.0	133	4.9
9	77	5.6	96	5.0	135	4.9
10	75	5.5	96	5.0	138	4.8
11	79	5.4	95	5.0	132	4.8
12	78	5.4	95	5.0	134	4.8
15	75	5.3	92	5.0	130	4.6

It can be seen from the table above that all three hypochlorous acid solutions were stable for at least 15 months. Notably, a 140 ppm stable hypochlorous acid solution is able to be prepared and stored for at least 15 months. This is a significant advance for the technical field. The hypochlorous acid solution prepared by the process of the present disclosure does not need to be buffered to reach the desired concentration, pH, purity and stability. The exemplified solutions had 99.5% or more HOCI in the solution, the % being defined as FAC.
The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future.

Claims

A continuous process for preparing a stable hypochlorous acid solution by electrolysis, wherein the process comprises:
providing a hydrochloric acid feed at a flow rate of about 2 ml/minute to about 6 ml/minute;

providing a water feed at a pressure of about 7 psi to less than about 15 psi;

adding the hydrochloric acid feed to the water feed to form a reactant solution feed;

measuring the pH of the reactant solution and subsequently providing the reactant solution feed to a single chamber electrolysis cell, the chamber comprising an anode and a cathode;

applying a voltage to the electrolysis cell to generate a current of about 5 A to about 20 A; and

measuring the pH of the hypochlorous acid solution as it exits the single chamber electrolysis cell;

wherein at least one of the anode or cathode are platinum-coated; and

wherein the electrical current, the hydrochloric acid feed flow rate, the water feed pressure and the reactant solution feed flow rate are monitored in real-time and controlled so that the pH of the reactant solution is up to about 3.5 and the pH of the hypochlorous acid solution is from about 4.0 to about 6.0.
The process according to claim 1, wherein the process is free from buffering agent and/or wherein the hydrochloric acid and water are the sole reactants provided to the single chamber electrolysis cell.
The process according to claim 1 or claim 2, wherein the electrolysis cell does not include a separating membrane.
The process according to any one of claims 1 to 3, wherein both the cathode and the anode are platinum-coated.
The process according to any one of claims 1 to 4, wherein the current is between about 5 A and about 15 A.
The process according to any one of claims 1 to 5, wherein the reactant solution feed flow rate is controlled between about 0.4 and about 2.5 l/min.
The process according to any one of claims 1 to 6, wherein the hydrochloric acid feed flow rate is from 2 to 4 ml/min.
The process according to any one of claims 1 to 7, wherein the water pressure is controlled between about 7 to about 12 psi.
The process according to any one of claims 6 to 8, wherein the reactant solution feed flow rate is controlled between about 0.4 to about 1.5 l/min.
The process according to any one of claims 1 to 9, wherein the quantity of hypochlorous acid produced is between about 20 and about 120 l/h, preferably between about 24 and about 90 l/h.
The process according to any one of claims 1 to 10, wherein the pH of the reactant solution is from about 2.5 to about 3.5.
The process according to any one of claims 1 to 11, wherein the pH of the hypochlorous acid solution is from about 4.5 to about 5.7.
The process according to any one of claims 1 to 12, wherein the hydrochloric acid feed is injected at a set pressure and flow rate into the water feed, preferably the hydrochloric acid feed is injected dropwise.
The process according to any one of claims 1 to 13, wherein the process is free from sodium hypochlorite.
A hypochlorous acid solution obtained by the process according to any one of claims 1 to 14.
The hypochlorous acid solution according to claim 15, wherein the concentration of hypochlorous acid in the solution is greater than 100 ppm, preferably wherein the concentration of hypochlorous acid is from about 110 ppm to about 140 ppm.
The hypochlorous acid solution according to claim 15 or claim 16, wherein the solution has a pH of about 4.8 and the hypochlorous acid is at a concentration of about 140 milligrams per liter.
An apparatus for preparing a stable hypochlorous acid solution, the apparatus comprising:
a single chamber electrolysis cell containing an anode and a cathode, at least one of the anode and the cathode being platinum-coated;

a water conduit and a hydrochloric acid conduit configured to provide a reactant solution feed to the single chamber electrolysis cell, wherein the hydrochloric acid conduit is connected to the water conduit via a first non-return valve, and the reactant solution feed is provided by a conduit connected to the single chamber electrolysis cell via a second non-return valve;

a hypochlorous acid solution conduit from the single chamber electrolysis cell;

a pH meter connected to the reactant solution conduit;

a pH meter connected to the hypochlorous acid solution conduit; and

a controller which is electrically connected to the electrolysis cell, the water conduit, the hydrochloric acid conduit, the reactant solution conduit and each of the pH meters;

wherein the controller includes a means to monitor, in real-time, the electrical current of the electrolysis cell, the hydrochloric acid flow rate, the water pressure, the reactant solution flow rate, the reactant solution pH and the hypochlorous solution pH, and a means to control one or more of the electrical current, the hydrochloric acid flow rate, the water pressure and the reactant solution flow rate so that the pH of the reactant solution is up to about 3.5 and the pH of the hypochlorous acid solution is from about 4.0 to about 6.0.
The apparatus according to claim 18, further comprising a cooling system for the single chamber electrolysis cell, preferably wherein the cooling system comprises air flow external to the electrolysis cell.
The apparatus according to claim 18 or claim 19, wherein each of the anode and cathode are platinum-coated.
The apparatus according to any one of claims 18 to 20, wherein the controller further comprises a means to monitor in real-time the water flow rate and a means to control the water flow rate so that the pH of the reactant solution and the pH of the hypochlorous acid solution are within the ranges defined in claim 18.
The apparatus according to any one of claims 18 to 21, wherein the hydrochloric acid conduit includes a dosing pump and a third non-return valve, wherein the first non-return valve is fitted adjacent to the outlet of the pump and the third non-return valve is fitted adjacent to the inlet of the pump.